1. Field of the Invention
The present invention relates to a reduced pressure or vacuum boiler type evaporator and in particular, to an evaporator including a vacuum boiler vessel and a condenser vessel arranged in a separate fashion, with means for collecting noncondensable gas so as not to deteriorate heat exchange effect.
2. Description of the Related Art
LNG evaporators include an open rack type evaporator wherein sea water as a heating medium is brought into contact with an array of vertical heat transfer tubes so as to increase the temperature of liquefied natural gas (LNG) to change from its liquid phase to its gaseous phase.
Another type of evaporator uses a burner as a heating source. A heat exchanger is immersed in a water vessel. The burner provides high temperature combustion gas below the heat exchanger. Water is used as an intermediate heating medium to increase the temperature of LNG. LNG is then vaporized to natural gas (NG).
However, such type of evaporator often suffers from malfunction of the burner or the burner fails to ignite fuel as it is placed in the water vessel. The heat exchanger is designed to accommodate vigorous circulating water. However, this results in complication of the heat exchanger and thus, in an increase in its production cost. A further problem is that large blower and motor are required to produce and maintain the stirring water.
A vacuum boiler also uses heat by means of a burner (see Japanese patent publication No. 47083/77, Japanese laid-open patent publication No. 245003/87 and Japanese patent publication No. 2667/87).
A typical vacuum boiler is illustrated in FIG. 3 and includes a vacuum vessel 1 within which water 2 as a heating medium is contained, a burner pipe 4 extending from the lower portion of the vacuum vessel 1 and adapted to transmit heat from a burner 3 to the water 2, and a heat transfer tubes 5 extending into the upper portion of the vacuum vessel 1. The water 2 is boiled under vacuum and vaporized to steam. Heat transfer of steam condensation is used to increase the temperature of fluid flowing through the heat exchange pipe 5. The vacuum boiler can be applied in various ways such as a hot water feed boiler or a heater.
When such a vacuum boiler is used as a low temperature fluid or LNG noncondensable gas results in a substantial decrease in the capability of the boiler.
Referring specifically to FIG. 3, the heat transfer tubes 5 is coupled to a LNG feed pipe 6, and the vacuum boiler functions as a LNG evaporator. Noncondensable gas may results in a decrease in the effectiveness of heat transfer between steam and LNG. Particularly, when a considerable amount of noncondensable gas is attached to the surface of the heat transfer tubes 5, ice is produced on its surface to deteriorate heat transfer between steam and LNG. LNG can not completely be vaporized, and a mixture of gas and low temperature liquid is disadvantageously delivered from the evaporator.
To this end, a vacuum pump 7 is required to remove noncondensable gas from the vacuum boiler type evaporator.
Accordingly, it is an object of the present invention to provide a vacuum boiler type evaporator which cooperates with a vacuum pump to effectively remove noncondensable gas or prevent attachment of noncondensable gas to the surface of a heat exchange pipe which may, otherwise, deteriorate the capability of the evaporator.